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The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear both to masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been observed among 1,500 species, and in 500 of those it is well documented.[1]

Contents

Mating systems

In sociobiology and behavioural ecology, the term mating system is used to describe the ways in which animal societies are structured in relation to sexual behaviour. The mating system specifies what males mate with what females under what circumstances.

The following are some of the mating systems generally recognised in humans and other animals:

Monogamy: One male and one female have an exclusive mating relationship.

Polygamy: One or more males have an exclusive relationship with one or more females. Three types are recognised:

Polygyny (the most common polygamous mating system in vertebrates so far studied): One male has an exclusive relationship with two or more females

Polyandry: One female has an exclusive relationship with two or more males.

Polygynandry: Two or more males have an exclusive relationship with two or more females; the numbers of males and females need not be equal, and in vertebrate species studied so far, there are usually fewer males.

Monogamy

Zoologists and biologists now have solid evidence that monogamous pairs of animals are not always sexually exclusive. Many animals that form pairs to mate and raise offspring regularly engage in sexual activities with extra-pair partners[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]
This includes previous exemplars such as swans and (depending upon circumstances) wolves. Sometimes these extra-pair sexual activities lead to offspring. Genetic tests frequently show that some of the offspring raised by a monogamous pair come from the female mating with an extra-pair male partner.[4][5][17][18]
These discoveries have led biologists to adopt new ways of talking about monogamy:

"Social monogamy refers to a male and female's social living arrangement (e.g., shared use of a territory, behaviour indicative of a social pair, and/or proximity between a male and female) without inferring any sexual interactions or reproductive patterns. In humans, social monogamy equals monogamous marriage. Sexual monogamy is defined as an exclusive sexual relationship between a female and a male based on observations of sexual interactions. Finally, the term genetic monogamy is used when DNA analyses can confirm that a female-male pair reproduce exclusively with each other. A combination of terms indicates examples where levels of relationships coincide, e.g., sociosexual and sociogenetic monogamy describe corresponding social and sexual, and social and genetic monogamous relationships, respectively." (Reichard, 2003, page 4)[19]

Whatever makes a pair of animals socially monogamous does not necessarily make them sexually or genetically monogamous. Social monogamy, sexual monogamy, and genetic monogamy can occur in different combinations.

Social monogamy is relatively rare in the animal kingdom. The actual incidence of social monogamy varies greatly across different branches of the evolutionary tree. Over 90 percent of avian species are socially monogamous.[13][20] This stands in contrast to mammals. Only 3 percent of mammalian species are socially monogamous, although up to 15 percent of primate species are socially monogamous.[13][20] Social monogamy has also been observed in reptiles, fish, and insects.

Sexual monogamy is very rare among animals. The great majority of socially monogamous species engage in extra-pair copulations, making them sexually non-monogamous. Gowaty has estimated that, out of 180 different species of socially monogomous songbirds, only 10 percent are sexually monogamous.[21]

The incidence of genetic monogamy, determined by DNA fingerprinting, varies widely across species. For a few rare species, the incidence of genetic monogamy is 100 percent, with all offspring genetically related to the socially monogamous pair. But genetic monogamy is strikingly low in other species. Barash and Lipton note:

"The highest known frequency of extra-pair copulations are found among the fairy-wrens, lovely tropical creatures technically known as Malurus splendens and Malurus cyaneus. More than 65 percent of all fairy-wren chicks are fathered by males outside the supposed breeding group." (Barash & Lipton, 2001, page 12)[20]

Such low levels of genetic monogamy have surprised biologists and zoologists, forcing them to rethink the role of social monogamy in evolution. They can no longer assume social monogamy determines how genes are distributed in a species. The lower the rates of genetic monogamy among socially monogamous pairs, the less of a role social monogamy plays in determining how genes are distributed among offspring. See also Evolution of Monogamy.

Polygyny

In some species, notably those with harem male structures, only one of few males in a group of females will mate. This is also known as polygyny in sociobiology.[22] Should the active male be driven out, in a number of species the new male will ensure that breeding resources are not wasted on another males' young.[23] These can include:

Competitive infanticide - in lions and some monkeys the new male will kill other young, therefore the females, no longer nursing, rapidly become receptive again.

Pheromone based spontaneous abortion - in some rodents such as mice, a new male with a different scent will cause females who are pregnant to spontaneously fail to implant recently fertilized eggs. This does not require contact; it is mediated by scent alone. It is known as the Bruce-Parkes effect.

Promiscuity

Two examples of systems in primates are promiscuous mating chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each male copulates with many females, and vice versa. In bonobos, the amount of promiscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce.

Seasonal nature of animal sexuality

Many animal species have specific mating (or breeding) seasons. These are often associated with changes to herd or group structure, and behavioral changes, including territorialism amongst individuals. These may be annual (eg wolves), biannual (eg dogs) or more frequently (eg horses). During these periods, females of most species are more mentally and physically receptive to sexual advances, a period often described as being "in season" or "in heat", but outside them animals still engage in sexual behaviors,[24] and such acts as do occur are not necessarily harmful.[25]

Interpretation of animal sexuality

The field of study of sexuality in non-human species has been a long standing taboo,[1] with researchers either failing to observe or mis-categorizing and mis-describing sexual behavior which does not meet their preconceptions. (See: Observer bias) More current research provides views such as that of the Norwegian Natural History Museum, which in 2006 held an exhibition on animal sexuality:

"Many researchers have described homosexuality as something altogether different from sex. They must realise that animals can have sex with who they will, when they will and without consideration to a researcher's ethical principles." [1]

An example of overlooking behaviour relates to descriptions of giraffe mating:

When nine out of ten pairings occur between males, "[e]very male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only [categorized as] 'revolving around' dominance, competition or greetings." [1]

Sex for pleasure

It is a common myth that animals do not (as a rule) have sex for pleasure, or alternatively that humans (and perhaps dolphins and one or two species of primate) are the only species which do. This is sometimes formulated "animals mate only for reproduction".

Science cannot say at present conclusively what animals do or do not find "pleasurable", a question considered in more depth under Emotion in animals. The urban myth site Snopes.com considers this particular view in depth. Its conclusions are broadly that the statement is true, but only using a very specific definition of "sex for pleasure" [italics in original], in which sexual acts tied to a reproductive cycle or for which an alternative explanation can be asserted, are ignored, as is all sexual activity that does not involve penetration. Animals put themselves at risk to engage in sex, and as a result, most species have evolved sexual signals (usually scent and behavior) to indicate the presence of receptive periods. During these, sex is sought, and outside these it is usually not sought (or is sought but not permitted). Snopes comments that this is not in fact a reflection of whether sex is pleasurable or not, but rather a reflection of whether individuals have sex at arbitrary times. They conclude[26]:

"Of course, we have to make many seemingly artificial distinctions to arrive at our conclusion. Animals other than humans have no awareness that their sexual activities are connected with reproduction: They engage in sex because they're biologically driven to do so, and if the fulfillment of their urges produces a physical sensation we might appropriately call 'pleasure,' it isn't the least bit affected by the possibility (or impossibility) of producing offspring. We are also discounting cases in which animals do engage in sex even though reproduction is an impossibility because we claim there are other 'purposes' (of which the animals themselves are unaware) at play. (For example, the females of some species of birds will invite males to mate with them even after they have laid their eggs, but we ascribe a purpose to this behavior: this is a biological "trick" to fool males into caring for hatchlings they didn't father.) We also employ subjective terms such as 'willingly' and 'regularly' in claiming that bonobos and dolphins are the only other animals who "willingly (and regularly) engage in sex with each other" ... and even then it may be the case that these species have some other 'purpose' for doing so that we haven't yet discovered..."

A 2006Danish Animal Ethics Council report[27] which examined current knowledge of animal sexuality in the context of legal queries concerning sexual acts by humans, has the following comments, primarily related to domestically common animals:

"Even though the evolution-related purpose of mating can be said to be reproduction, it is not actually the creating of offspring which originally causes them to mate. It is probable that they mate because they are motivated for the actual copulation, and because this is connected with a positive experience. It is therefore reasonable to assume that there is some form of pleasure or satisfaction connected with the act. This assumption is confirmed by the behavior of males, who in the case of many species are prepared to work to get access to female animals, especially if the female animal is in oestrus, and males who for breeding purposes are used to having sperm collected become very eager, when the equipment they associate with the collection is taken out."[28]

"There is nothing in female mammals’ anatomy or physiology, that contradicts that stimulation of the sexual organs and mating is able to be a positive experience. For instance, the clitoris acts in the same way as with women, and scientific studies have shown that the success of reproduction is improved by stimulation of clitoris on (among other species) cows and mares in connection with insemination, because it improves the transportation of the sperm due to contractions of the inner genitalia. This probably also concerns female animals of other animal species, and contractions in the inner genitals are seen e.g. also during orgasm for women. It is therefore reasonable to assume that sexual intercourse may be linked with a positive experience for female animals."[29]

Types of activity

Autoeroticism (masturbation)

It appears that many animals, both male and female, masturbate, both when partners are available and otherwise.

For example, http://www.petplace.com comments in its guide on assessing potential breeding stock purchases: "Masturbation is a normal behavior in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion."[30] Likewise the paper "Sexual Behavior - Current Topics in Applied Ethology and Clinical Methods" by Sue McDonnell of the Equine Behavior Laboratory, University of Pennsylvania School of Veterinary Medicine states:

"One example is the behavior known within the horse breeding industry as masturbation. This involves normal periodic erections and penile movements. This behavior, both from the descriptive field studies cited above and in extensive study of domestic horses, is now understood as normal, frequent behavior of male equids.[31]

Attempting to inhibit or punish masturbation, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behavior (McDonnell and Hinze, in preparation).[32]

Sexual release seeking is common in both domestic and non-domestic species. For example, a video (non-explicit) showing a kangaroo masturbating, inadvertently caught during a TV broadcast, can be found here. Also, another video (explicit) of a kangaroo performing autofellatio can be found here, and a dog balancing on rear legs in order to masturbate with both front paws, here.

The female porcupine will use a stick as a vibrator, holding one end of a stick between her paws and walk around, straddling the stick as it bumps against the ground and vibrates against her genitalia.[33] Sexologist Havelock Ellis in his 1927 "Studies in the Psychology of Sex" identified bulls, goats, sheep, camels and elephants as species known to practice autoeroticism, adding of some other species:

I am informed by a gentleman who is a recognized authority on goats, that they sometimes take the penis into the mouth and produce actual orgasm, thus practicing auto-fellatio. As regards ferrets ... "if the bitch, when in heat, cannot obtain a dog [ie, male ferret] she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it." [...] Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other's genitals.

Autoeroticism also occurs widely among animals, both male and female. A variety of creative techniques are used, including genital stimulation using the hand or front paw (primates, Lions), foot (Vampire Bats, primates), flipper (Walruses), or tail (Savanna Baboons), sometimes accompanied by stimulation of the nipples (Rhesus Macaques, Bonobos); auto-fellating or licking, sucking and/or nuzzling by a male of his own penis (Common Chimpanzees, Savanna Bonobos, Vervet Monkeys, Squirrel Monkeys, Thinhorn Sheep, Bharal, Aovdad, Dwarf Cavies); stimulation of the penis by flipping or rubbing it against the belly or in its own sheath (White-tailed and Mule Deer, Zebras and Takhi); spontaneous ejaculations (Mountain Sheep, Warthogs, Spotted Hyenas); and stimulation of the genitals using inanimate objects (found in several primates and cetaceans).[34]

Many birds masturbate by mounting and copulating with tufts of grass, leaves or mounds of earth, and some mammals such as primates and Dolphins also rub their genitals against the ground or other surfaces to stimulate themselves.[34]

Autoeroticism in female mammals, as well as heterosexual and homosexual intercourse (especially in primates), often involves direct or indirect stimulation of the clitoris [...]. This organ is present in the females of all mammalian species and several other animal groups.[34]

and that:

Apes and Monkeys use a variety of objects to masturbate with and even deliberately create implements for sexual stimulation [...] often in highly creative ways.[34]

Petter Boeckman of the Norwegian Natural History Museum commented (in respect of a 2006 exhibition on sexuality in the animal kingdom) that:

"Masturbation is common in the animal kingdom ... We have a Darwinist mentality that all animals only have sex to procreate. But there are plenty of animals who will masturbate when they have nothing better to do. Masturbation has been observed among primates, deer, killer whales and penguins, and we're talking about both males and females. They rub themselves against stones and roots. Orangutans are especially inventive. They make dildos of wood and bark." [1]

Oral sex

Animals of several species are documented as engaging in both auto-fellatio and oral sex. Although easily confused by lay-people, this is a separate and sexually oriented behavior, distinct from non-sexual grooming or scent-investigation.

Homosexual behavior

The presence of same-sex sexual behavior was not scientifically observed on a large scale until recent times. Homosexual behaviour does occur in the animal kingdom outside humans, especially in social species, particularly in marine birds and mammals, monkeys, and the great apes. Homosexual behaviour has been observed among 1,500 species, and in 500 of those it is well documented.[35]

"To turn the approach on its head: No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue." [1]

Georgetown University professor Janet Mann has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimizes intraspecies aggression, especially among males.

Male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In 2004, the Central Park Zoo in the United States replaced one male couple's stone with a fertilized egg, which the couple then raised as their own offspring.[36]German and Japanesezoos have also reported homosexual behaviour among their penguins. This phenomenon has also been reported at Kelly Tarlton's Aquarium in Auckland, New Zealand. "Humans have created the myth that sexuality can be justified only by reproduction, which by definition limits it to hetero sex," says Michael Bronski, author of The Pleasure Principle: Culture, Backlash, and the Struggle for Gay Freedom. "But here is an animal society that uses homosexuality to improve its social life."

Courtship, mounting, and full anal penetration between bulls has been noted to occur among American Bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behaviour, to "ensure the return of the buffalo in the coming season."[37] Also, mounting of one female by another is common among cattle. (See also, Freemartin. Freemartins occur because of clearly causal hormonal factors at work during gestation.)

Bonobos in zoos. After studying the primates for his book Bonobo: The Forgotten Ape, primatologist Frans de Waal, a professor of psychology at Emory University in Atlanta, says that such expressions of intimacy are consistent with the homosexual behavior of what he terms "the erotic champions of the world." "Same-sex, opposite-sex — bonobos just love sex play," de Waal said in an interview. "They have so much sex, it gets boring."

Homosexual behaviour in male sheep (found in 6-10% of rams) is associated with variations in cerebral mass distribution and chemical activity. A study reported in Endocrinology concluded that biological and physiological factors are in effect.[38] These findings are similar to human findings reported by Simon LeVay.

"Approximately eight percent of [male] rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes..."

Male bighorn sheep are divisible into two kinds, the typical males among whom homosexual behavior, including intercourse, is common and "effeminate sheep" or "behavioral transvestites" which are not known to engage in homosexual behavior.[39][40]

Cross species sex

Although many people believe animal sexuality is instinctive and therefore (it is implied) almost mechanistic, in fact research regularly records that many animals are sexual opportunists, and may show an interest in partners other than their own or related species. [How to reference and link to summary or text] This is more visible in domesticated species, as domestication commonly selects for increased breeding rate (and so an accelerated breeding cycle has commonly arisen in domesticated species over the centuries), and also because these species are more easily observed by humans. Nevertheless, non-domesticated animals have been observed to attempt sexual activity with other species or indeed inanimate objects. Such cross-species sex has been observed more with animals in captivity than in the wild, probably due to ease of observation, although attempts by wild moose to obtain sex from domestic horses are apparently well known by wildlife specialists.[41]

In the wild, where observation is harder, genetic studies have shown a "large number" of inter-species hybrids, and other investigations describe productive and non-productive inter-species mating as a "natural occurrence".[42] Recent genetic evidence strongly suggesting this has occurred even within the history of the human species, and that early humans often had sexual activity with other primate species, is considered below.

Hybrid offspring can result if the two species are related. However, this offspring may not necessarily be fertile itself. The mule, for example (a horse/donkey cross) is normally sterile, whilst the liger (lion/tiger cross) has fertile females and sterile males. Novosibirsk zoo director Rostislav Shilo says on the liger (born in its zoo); "It’s just that the lion and the tiger live in neighboring caves in the Novosibirsk zoo, and got used to each other. It’s practically impossible in the wild."

Due to the difficulties of observation, interspecies sex of this kind between two top-level predators, occurring in the wild, was only conclusively documented with the finding of a grizzly-polar bear hybrid in April 2006. Again, as with lions and tigers, the two species would normally not share enough common territory to provide adequate opportunity for much cross-species sexual activity. In other words, whether both species were 0% or 100% promiscuous and sought sex with the same species only or the nearest bear of any species, the overwhelming number of matings would still of necessity be with the same species.

Animal sexual advances on, and attempted interactions with, humans and other species, have been documented by ethologists such as Kohler, Gerald Durrell and Desmond Morris, as well as authoritative researchers such as Birute Galdikas who studied orangutans in Borneo. Peter Singer, the philosopher and animal welfare activist reports: "While walking through the camp with Galdikas, my informant was suddenly seized by a large male orangutan, his intentions made obvious by his erect penis. Fighting off so powerful an animal was not an option, but Galdikas called to her companion not to be concerned, because the orangutan would not harm her, and adding, as further reassurance, that 'they have a very small penis.'" (though "the orangutan lost interest before penetration took place")[43]

Sexual fetishes

Although not often reported, it appears animals, or primates at the least, are able to sexualize inanimate objects in a manner similar to human sexual fetishes.

Not only will an animal that has a habitual object for masturbation sometimes appear to sexualize that object, but in some cases primates have generalized that to sexualize kinds of objects in a class where no prior sexual connection exists — similar to human fetishes.

In this case however, the primary structure is Pavlovian conditioning, and the fetishistic association is due to a conditioned response (or association) formed with a distinctive 'reward'. Human fetishism can also be traced back to similar or near-identical conditioning: likewise based upon the Pavlovian association between an erotic sensation or anticipation, and objects which become immediately associated with that activity. (See also: operant conditioning)

Washoe, a chimpanzee who has been taught American Sign Language, has been reported to frequently make the sign for "tickle me" to researchers. Although not a sexual act per se, tickling is nonetheless recognized as a fetish in some contexts. (See: Tickling fetish)

Sexual imagery viewing

A study by Platt, Khera and Deaner at Duke University North Carolina (reported in Current Biology and online here), showed that male monkeys will give up privileges (in this case, juice, which was highly valued), to be allowed to see a female monkey's hindquarters.[44]

Deaner and his team reported that monkeys would take a juice cut to look at powerful males' faces or the perineum of a female, but to persuade the monkeys to stare at subordinate males, the researchers had to bribe them with larger drinks. "Virtually all [male] monkeys will give up juice to see female hindquarters ... they really value the images."

The researchers stress that in monkey society, such behaviors have great social utility and we should therefore not simply reach the conclusion that "monkeys enjoy pornographic pictures". There is no evidence at this point that viewable pictures or movies of sexual activity are valued for their sexual enjoyment, although as noted above (Masturbation), there are reports that watching sex in real life may have such an effect. The subject of animals and sexual imagery is not yet well researched.

Problems with encouraging pandas to mate in captivity have been very common. However, showing young male pandas "panda pornography" is widely credited with a recent population boom among pandas in zoos.[45]

In 2007, research suggested that in the Aciliusgenus of water beetles (also known as "diving beetles"), an "evolutionary arms race" between the genders means that there is no courtship system for these beetles. "It's a system of rape. But the females don't take things quietly. They evolve counter-weapons." Cited mating behaviors include males suffocating females underwater till exhausted, and allowing only occasional access to the surface to breathe for up to six hours (to prevent them breeding with other males), and females which have a variety of body shapings (to prevent males from gaining a grip). Foreplay is "limited to the female desperately trying to dislodge the male by swimming frantically around."[46]

Charles Siebert reports in his New York Times article Elephant Crackup? that:

Since the early 1990’s, for example, young male elephants in Pilanesberg National Park and the Hluhluwe-Umfolozi Game Reserve in South Africa have been raping and killing rhinoceroses; this abnormal behavior, according to a 2001 study in the journal Pachyderm, has been reported in ‘‘a number of reserves’’ in the region.[47]

Sex between adults and juveniles

It has also been recorded that certain species of mole will impregnate newborns of their own species. [How to reference and link to summary or text] It is not clear if this is forceful or not. Similarly, the male stoat (Mustela erminea) will mate with infant females of their species.[48] This apparently is a natural part of their reproductive biology - there is a delayed gestation period, so these females give birth the following year when they are fully grown.

Sexual cannibalism

Sexual cannibalism, which has been documented in arachnids, insects and amphipods, is a phenomenon in which a female organism kills and consumes the male before, during, or after copulation. Although it does confer some known advantages to reproduction, whether or not the male is complicit has not been scientifically determined.

Necrophilia

Necrophilia in animals is where a living animal engages in a sexual act with a dead animal. In one of the most well-known examples, Kees Moeliker of the Rotterdam Natural History Museum, Netherlands observed sexual activities[49] outside his office between a live duck and a dead one.

While he was sitting in his office at the Natuurmuseum Rotterdam, Moeliker heard the distinctive thud of a bird hitting the glass facade of the building. Upon inspection, he discovered a drake mallard lying dead about two meters from the building. Next to the downed bird there was a second drake mallard standing close by. As he observed the odd couple, the living drake picked at the corpse of the dead one for a few minutes and then, without provocation, it mounted the corpse and began copulating with it. The act of necrophilia lasted for about 75 minutes, in which time, according to Moeliker, the living drake took two short breaks before resuming with copulating behavior. Moeliker surmised that at the time of the collision with the window the two mallards were engaged in a common motif in duck behavior which is called rape flight. "When one died the other one just went for it and didn't get any negative feedback—well, didn't get any feedback," according to Moeliker.

After the live bird was shooed away, inspection of the dead mallard revealed that it was male, thus making this the first observed case of homosexual necrophilia in mallards. The case was reported scientifically in Deinsea 8-2001, along with photos.[50] The paper also netted Moeliker an Ig Nobel Prize in biology awarded for improbable research.[51]

Birds

Some black swans of Australia form sexually active male-male mated pairs and steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. More of their cygnets survive to adulthood than those of different-sex pairs possibly due to their superior ability to defend large portions of land.

Zoos in Japan and Germany have also documented male penguin couples. The couples have been shown to build nests together and use a stone to replace an egg in the nest. Researchers at Rikkyo University in Tokyo, found twenty such pairs at sixteen major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to break up the male couples by importing female penguins from Sweden and separating the male couples; they were unsuccessful. The zoo director stated the relationships were too strong between the couples.

Recently, a mated pair of swans in Boston were found to both be female. They too had attempted to raise eggs together. (365gay.com)

Studies have shown that ten to fifteen percent of female western gulls in some populations in the wild prefer other females.

As many as 19% of Mallard pairs in a given population have been observed to consist of male-male homosexuals.[52]

Lizards

Whip-tailed lizard females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard. Females engage in sexual behavior to stimulate ovulation, with their behavior following their hormonal cycles; during low levels of estrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high estrogen levels assume "feminine" sexual roles.

Lizards that perform the courtship ritual have greater fecundity than those kept in isolation due to an increase in hormones triggered by the sexual behaviors. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success.

From an evolutionary standpoint these females are passing their full genetic code to all of their offspring rather than the 50% of genes that would be passed in sexual reproduction. Certain species of gecko also reproduce by parthenogenesis.

However, some view this study to be flawed in that the determination of homosexuality within the sheep, (sample population of twenty-seven for the study), was to have animals who were unable to mount female ewes placed in a cage with two stanchioned males and two unstanchioned females (that is, the males could not move or struggle while the females could). Given the aggressive nature of the sheep copulation, the uneven treatment of males and females, many see this as simply evidence that the sheep in question were unable to be aggressive enough to mount females. Some say that the results were situational sexuality, unlike the bonds seen in human homosexuality. However the physical brain anatomy of the rams that preferred males were different.

The scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol so that the androgenhormone can facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes."

"The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "...naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis." Read the abstract of the study. As noted previously, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. The results of this study have not been confirmed by others.

Spotted Hyena

The female Spotted Hyena has a unique urinary-genital system, closely resembling the penis of the male, called a pseudopenis. The family structure is matriarchal and dominance relationships with strong sexual elements are routinely observed between related females.

They are notable for using visible sexual arousal as a sign of submission and not dominance, in males as well as females (females have a sizable erectile clitoris), to the extent that biologist Robert Sapolsky speculates that in order to facilitate this, their sympathetic and parasympathetic nervous systems may be partially reversed in respect to their reproductive organs.[53]

Bottlenose Dolphins

Bottlenose Dolphin males have been observed working in pairs to follow and/or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. The same pairs have also been observed engaging in intense sexual play with each other.

Janet Mann, a professor of biology and psychology at Georgetown University, argues[54] that the common same-sex behavior among male dolphin calves is about bond formation and benefits the species evolutionarily. They cite studies that have shown the dolphins later in life are bisexual and the male bonds forged from homosexuality work for protection as well as locating females to reproduce with.

Seahorses

Seahorses, long upheld as monogamous and mating for life, are identified as "promiscuous, flightly, and more than a little bit gay" according to research published in 2007.[55]

Scientists at 15 aquariums studied 90 seahorses of 3 species. Of 3168 sexual encounters, 37% were same sex acts. Flirting was common (up to 25 potential partners a day of both genders); only one species (the British Spiny Seahorse) included faithful representatives, and for these 5 of 17 were faithful, 12 were not. Bisexuality was widespread and considered "both a great surprise and a shock", with big bellied seahorses of both genders not showing partner preference. 1986 contacts were male-female, 836 were female-female and 346 were male-male. [How to reference and link to summary or text]

Lions

Male lions often lead their social groups jointly with one or more of their brothers. To ensure loyalty, the male co-leaders will "strengthen the bonds by often having sex with each other." [1]

Other evidence of interspecies sexual activity

Looking back in history, current research into human evolution tends to confirm that in some cases, interspecies sexual activity may have been responsible for the evolution of entire new species. Analysis of human and animal genes in 2006 provides strong evidence that after humans had diverged from apes, interspecies mating nonetheless occurred regularly enough to change certain genes in the new gene pool:

"A new comparison of the human and chimpgenomes suggests that after the two lineages separated, they may have begun interbreeding. [...] A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes."

The research suggests that:

"There were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are "treating the new genetic data seriously."

The Washington Postcomments, "If this theory proves correct, it will mean modern people are descended from something akin to chimp-human hybrids."[56]

↑Hubrecht, R.C. (1985). Home range size and use and territorial behavior in the common marmoset, Callithrix jacchus jacchus, at the Tapacura Field Station, Recife, Brazil. International Journal of Primatology, 6, 533-550.

↑Mason, W.A. (1966). Social organization of the South American monkey, Callicebus moloch: a preliminary report. Tulane Studies in Zoology, 13, 23-28.

↑For example, masturbation, trial mounting, and other behaviors are regularly seen in male animals out of season

↑2006 Danish Animal Ethics Council report: "The mucous membrane in the female animal’s vagina and the animal’s behavior is under influence of its rut cycle. That means that the animal is physically and mentally more ready for sexual activities at some times than at others. But this does not mean that sexual activity will lead to injuries, fear or suffering, if it happens outside the rut period." (Danish: "Slimhinden i hundyrets vagina og dyrets adfærd er under indflydelse af dets brunstcyklus. Det betyder, at dyret er fysisk og mentalt mere parat til seksuelle aktiviteter på nogle tidspunkter end på andre. Men dette er ikke ensbetydende med, at den seksuelle aktivitet vil være forbundet med skader, angst og lidelse, hvis den foregår udenfor brunstperioden.")Report November 2006 (PDF)

↑Danish Animal Ethics Council reportUdtalelse om menneskers seksuelle omgang med dyr published November 2006. Council members included two academics, two farmers/smallholders, and two veterinary surgeons, as well as a third veterinary surgeon acting as secretary.

↑Haeberle (1978) states that sexual intercourse is not so very unusual between animals of different species as it is between humans and animals. Kinsey et al. (1948, p. 668) states "When one examines the observed cases of such crosses, and especially the rather considerable number of instances in which primates, including man, have been involved, one begins to suspect that the rules about intraspecific mating are not so universal as tradition would have it". Kinsey et al. (1953) further point out that genetic studies have shown the existence of a "large number" of inter-specific hybrids, that have occurred in the wild, and investigations (eg, Cauldwell, 1968; Ford & Beach, 1951; Harris, 1969; Masters, 1962; Ullerstam, 1966, etc) have found that interspecies mating is a "natural occurrence".' (Cited by Miletski, in her anthrozoological study of animal-human sexuality, 1999, p.51)